Ceramic and ceramic composite heating wires are brittle in comparison to metallic wires. Yet they are preferred to metallic heating elements on account of both a higher resistivity and a much higher temperature of possible use. Typical ceramic heating element wires are made out of MoSi.sub.2, SiC, WSi.sub.2, ZrO.sub.2, etc., sometimes composited with Al.sub.2 O.sub.3, SiO.sub.2, etc.
Metallic heating elements wires, on account of their ductility and lower temperatures of use, may often be coiled and embedded in refractories to facilitate easy application and unit wise repair. On account of their high ductility, the metallic heating wires (typically containing Ni, Cr, Al, Fe, etc.) may be easily configured into the required furnace design without much effort and thought.
On the other hand when designing furnaces with ceramic/ceramic composite heating elements (hereinafter called cerheaters) the design of the furnace configuration must take into account the following factors: 1. creep (deformation of an article at high temperatures under its own weight); 2. reactivity with refractories; 3. extreme brittleness of heating elements at low temperatures; 4. large change in resistivity with temperature; 5. atmosphere of use; 6. connectors; 7. thermal fatigue; 8. thermal expansion; 9. lorentz force repulsion. Thus it can be seen that the cerheater design is often very complex and involved.
The problem of design becomes even more severe in applications which require use of elements having thicknesses (i.e., the diameter of heating element wires) less than 12 mm, especially in the range of 1 to 2 min. At such small thickness, the brittleness problems and problems associated with creep, become more pronounced. Thin wires are preferred in furnace design because the amount of current required (electricity) and associated component costs significantly diminish for the same temperature and wattage of the furnace, for reduced thickness elements when compared to elements having larger thickness.
To date, the typical use of cerheaters made from, for e.g., MoSi.sub.2, have been in the hanging U configuration. As MoSi.sub.2 is prone to large expansion and creep (especially when the heating zone of the element is long), this configuration has been deemed most appropriate for use in high temperature furnaces. On account of the relative inflexibility of available shapes, prior to the methods of manufacture described in U.S. Ser. No. 07/847,782 ("the '782 application") filed Mar. 5, 1992 and U.S. Ser. No. 08/028,354 ("the '354 application") filed on Mar. 9, 1993, this design has remained the preferred design. These two applications are hereby incorporated by reference in their entirety.
Now on account of finer wires and complex shapes available using the methods of the '782 application and the '354 application, there is a need for better and more efficient designs for heating elements which avoid damage/breakage during use and during shipping. The present invention includes some such designs where thin wires may be used at medium to high temperatures while overcoming the problems of brittleness, thermal expansion, creep, contacts etc. There is also included in the present invention, an improvement in a method for joining electrical heating elements to terminals and supports, whereby the joined products are densified for effective robust use at high temperatures.